1. Field of the Invention
The present invention relates to power modules and especially to techniques for improving cooling performance of power modules.
2. Description of the Background Art
FIG. 34 is a schematic external view of a first conventional power module 101P. In the power module 101P, a copper base plate 9P is disposed through a heat-conducting grease (not shown) over a radiating fin or heat sink 2AP, and an insulating substrate 5P is disposed on the base plate 9P. On the insulating substrate 5P, there are disposed a freewheeling diode 1AP (hereinafter also referred to as “diode”) and an insulated gate bipolar transistor 1BP (hereinafter referred to as “IGBT”).
In the conventional power module 101P, copper foils 6P are placed on both main surfaces of the insulating substrate 5P. The base plate 9P and the copper foil 6P are bonded together with solder, and the diode 1AP and the IGBT 1BT are soldered onto the copper foil 6P. An electrode 3P is provided through an insulating layer 4P over the radiating fin 2AP. Then, predetermined electrical connections are made by wires 7P. The construction including the radiating fin 2AP, the diode 1AP, the IGBT 1BP, and the like is housed in a case (not shown).
The electrode 3P is connected to a bus bar or wiring 91P which extends toward the outside of the case. Outside the case, a current transformer 92P for current detection is attached to the bus bar 91P. Further, a cylindrical capacitor 8P for smoothing direct current is provided outside the case independently of the radiating fin 2P and the like (the connection with the case is omitted in the figure).
FIG. 35 is a schematic external view of a second conventional power module 102P. The power module 102P has no base plate 9P as above described, wherein the insulating substrate 5P is disposed through a heat-conducting grease over the radiating fin 2AP. The power module 102P is in all other aspects identical to the above-mentioned power module 101P.
FIG. 36 is a schematic external view of a third conventional power module 103P. The power module 103P is a so-called power transducer. In the power module 103P, all the diodes 1AP and IGBTs 1BP are disposed on the insulating substrates 5P. A heat sink 2BP of the power module 103P has through holes 2BHP therethrough passing a cooling medium. The power module 103P is in all other aspects identical to the above-mentioned power module 101P.
The conventional power modules 101P, 102P, and 103P have the following problems.
First is low temperature reliability during operation. More specifically, when the thermal expansion coefficient of the heat sink 2AP or 2BP differs from those of the diode(s) 1AP and the IGBT(s) 1BP, thermal stresses responsive to a temperature difference from the freezing point of solder will occur at the solder joints as above described. There is thus a problem of occurrence and progress of cracking at the solder joints through a heat cycle (or temperature cycle) in the use (or operation) of the power module 101P, 102P, 103P and/or a heat cycle by repetitions of start and halt of the power module. Such cracking at the solder joints reduces the longevity of the power module.
To reduce the above thermal stresses, it is contemplated for example to increase solder thickness (e.g., 300 μm or more). However, such increased thickness of solder increases thermal resistance between the heat sink 2AP or 2BP and the diode(s) 1AP and the like. This brings up another problem that the size of the heat sink 2AP or 2BP must be increased.
Further, in the conventional power modules 101P, 102P, and 103P, the distribution of temperature in the insulating substrate(s) 5P, the base plate 9P, and the like due to heat generation in the diode(s) 1AP and the like causes warps or winding in the insulating substrate(s) 5P and the like. When the temperature difference is great, clearance is created between the radiating fin 2AP, 2BP and the base plate 9P and the like. Thus, there is a problem of reduced heat transfer because the heat-conducting grease cannot completely fill in the space between the radiating fin 2AP, 2BP and the insulating substrate(s) 5P or the base plate 9P (due to the incoming air). Another problem is that the occurrence or progress of cracking at the solder joints, described above, may be encouraged. The formation of clearance thus results in deterioration in the reliability of the power module.
To prevent the formation of clearance, it is contemplated for example to make the temperature distribution uniform throughout the insulating substrate(s) 5P and the like, or to increase the rigidity of the insulating substrate(s) 5P and the like by increasing the thickness of the substrate(s) 5P and the like. However, such increased thickness increases thermal resistance between the heat sink 2AP, 2BP and the insulating substrate(s) 5P or the like. This brings up, as has been described, another problem that the size of the heat sink 2AP, 2BP must be increased.
Further, when the diode(s) 1AP and the IGBT(s) 1BP produce a large quantity of heat, the amount of current must be limited in order to ensure reliability since the characteristics of the elements vary with increasing temperature.
Secondly, each of the conventional power modules 101P, 102P, and 103P as a whole is large in size since the current transformer 92P and the cylindrical capacitor 8P are provided independently outside the case for such a module. Besides, the current transformer 92P has the property of becoming large when current to be measured has a large DC component, and also the current transformer 92P makes measurements with errors (about 5%) due to its characteristics changes caused by heat generation.
Thirdly, in the power module 103P, the distances from each of the power semiconductor devices, such as the diode 1AP or the IGBT 1BP, to the electrode 61P connected to the low potential side of the power transducer and to the electrode 62P connected to the high potential side vary according to where that power semiconductor device is located. This causes variations in the inductance of the wiring or wires 7P from one power semiconductor device to another, thereby causing variations in output voltage.